The present invention relates to a communication apparatus enabling an Intranet connection in a network where different communication protocols mixedly exist.
At present, shifting from IPv4 (Internet Protocol version 4) to IPv6 which has a vast address space in order to solve a problem that global addresses of IPv4 will dry up has attracted attention. However, most of communications have been heretofore made using IPv4, and accordingly, it is substantially impossible to change all the communications instantaneously to communication with IPv6.
Further, among apparatuses which have already operated and have been installed with IPv4, some apparatuses require cost for expansion for the purpose of enabling such IPv6 communication, and cannot substantially perform a change to enable the communication by IPv6.
Therefore, in order to shift IPv4 to IPv6, a method has been adopted, which gradually replaces the current apparatuses with an apparatus and a relay apparatus, which are capable of the IPv6 communication. As a result, IPv4 and IPv6 mixedly exist on an IP network.
There are various shifting technologies enabling the IPv6 communication on the conventional IPv4 network in the case of such shifting to IPv6. One of those technologies is a tunnel technology, in which an IPv6 packet is encapsulated by an IPv4 header, communication is made by using the IPv4 header in an IPv4 network, and on a network or a terminal, which is capable of the IPv6 communication, the IPv4 header used for the encapsulation is detached to return (decapsulate) the encapsulated packet to the IPv6 packet.
Further, there are also various methods in this tunnel technology. As one of the methods, there is an automatic tunnel method utilizing 6to4 tunneling protocol. In a 6to4 tunnel, into an IPv6 address of an IPv6 header in an IPv6 packet sent out from a terminal installed with IPv6, which is a transmission source (hereinafter, referred to as “IPv6 terminal”), an IPv4 global address used when the IPv6 packet is encapsulated by IPv4 is embedded.
When the IPv6 packet passes through the IPv4 network, a terminal and a relay apparatus, which are located on an interface between IPv4 and IPv6, take out the IPv4 global address from the IPv6 address. Then, based on the taken-out IPv4 global address, the IPv4 header is automatically created, and communications with the IPv6 packet are thus made possible in the IPv4 network. Specifically, the 6to4 tunnel is one of automatic tunnel technologies.
In order to use this 6to4 tunnel, the IPv6 communication is performed by using an IPv6 address of a format in which higher-order 16 bits of the IPv6 address are a fixed value which is 2002 in the hexadecimal numbering system and the IPv4 global address is embedded into 32 bits following this fixed value.
The IPv4 header for encapsulation can be automatically created from the IPv4 global address embedded in such a way. However, in the 6to4 tunnel, there is a regulation that this embedded IPv4 address must be a global address (described in Chapter 2 of RFC3056 (Non-Patent document 1) which defines the 6to4 tunnel).
As defined in RFC 1918, IPv4 private addresses are addresses having prefixes like 10/8, 172.16/12 and 192.168/16, and in any organization, these addresses can be freely used. Therefore, a case normally occurs, where terminals having the same private address exist in plural different organizations.
Meanwhile, in an IPv4 public network such as the Internet, a private address cannot be used, and a global address must be used. This is because, when an arbitrary private address is used on the Internet, an address of a terminal overlaps the others, and the terminal cannot be identified. In general, the tunnel of the 6to4 format is constructed on the IPv4 public network, and accordingly, the private address has not been usable for the IPv4 address also in the case of using the tunnel of the 6to4 format.
Further, as known technologies for performing the encapsulation between the different communication protocols and tunneling by using the private address between private networks, the following are cited.
Patent document 1 discloses a technology for encapsulating, by a 6to4 tunneling encapsulator, a communication between IPv6 domains separated by IPv4 domains.
Patent document 2 discloses a technology, in which nodes which support incompatible network layer protocols, such as IPv4 and IPv6, determine a protocol set and perform automatic encapsulation therefor.
Patent document 3 discloses a technology for directly interconnecting the private networks through the tunneling by using private IP addresses.
[Patent document 1] JP 2003-510904 A
[Patent document 2] JP 2004-515165 A
[Patent document 3] JP 2003-273935 A
[Non-Patent document 1] B. Carpenter and another, “RFC3056”, [online], February 2001, [retrieved on Dec. 8, 2004], Internet <URL: http://rfc.net/rfc3056.html>
In the conventional techniques, there is a possibility that a packet using a private address is transmitted to the public network in the tunnel technology such as a 6to4 tunnel technology, and accordingly, the private address has not been usable.